Re: [PATCH v3] net: sock: move ->sk_shutdown out of bitfields.

[Eric Dumazet]

On Wed, 2016-05-18 at 17:36 +0300, Andrey Ryabinin wrote:
> ->sk_shutdown bits share one bitfield with some other bits in sock struct,
> such as ->sk_no_check_[r,t]x, ->sk_userlocks ...
> sock_setsockopt() may write to these bits, while holding the socket lock.
> 
> In case of AF_UNIX sockets, we change ->sk_shutdown bits while holding only
> unix_state_lock(). So concurrent setsockopt() and shutdown() may lead
> to corrupting these bits.
> 
> Fix this by moving ->sk_shutdown bits out of bitfield into a separate byte.
> This will not change the 'struct sock' size since ->sk_shutdown moved into
> previously unused 16-bit hole.
> 
> Signed-off-by: Andrey Ryabinin <aryabi...@virtuozzo.com>
> Suggested-by: Hannes Frederic Sowa <han...@stressinduktion.org>
> ---
> 
> Changes since v1:
>  - move sk_shutdown into a separate byte instead of locking
>       AF_UNIX sockets.
> 
> Changes since v2:
>  - reorder bitfields, so that sk_type/sk_protocol still fit in
>     separate 2-bytes/byte.
>       

I suggested to use an explicit padding, to clearly show that two bits
are available there. You also could add a comment to remind the rules.

Something like :

diff --git a/include/net/sock.h b/include/net/sock.h
index 
c9c8b19df27c558354687119db60c0716909ea3f..4e239a7ef4f67b1988fe119d81566d1465a2b596
 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -382,8 +382,10 @@ struct sock {
        atomic_t                sk_omem_alloc;
        int                     sk_sndbuf;
        struct sk_buff_head     sk_write_queue;
+
+       /* because of non atomicity rules, all changes are protected by socket 
lock */
        kmemcheck_bitfield_begin(flags);
-       unsigned int            sk_shutdown  : 2,
+       unsigned int            sk_padding   : 2,
                                sk_no_check_tx : 1,
                                sk_no_check_rx : 1,
                                sk_userlocks : 4,
@@ -391,6 +393,7 @@ struct sock {
                                sk_type      : 16;
 #define SK_PROTOCOL_MAX U8_MAX
        kmemcheck_bitfield_end(flags);
+
        int                     sk_wmem_queued;
        gfp_t                   sk_allocation;
        u32                     sk_pacing_rate; /* bytes per second */
@@ -418,6 +421,7 @@ struct sock {
        struct timer_list       sk_timer;
        ktime_t                 sk_stamp;
        u16                     sk_tsflags;
+       u8                      sk_shutdown;
        u32                     sk_tskey;
        struct socket           *sk_socket;
        void                    *sk_user_data;